(1) Field of the Invention
The present invention relates to a surface protection material for printed circuit boards and also a process for forming surface protection films, and more particularly to a surface protection material used for protection treatment on circuit sections composed of copper or copper alloys, and a process of forming surface protection films using such a surface protection material.
(2) Description of the Related Art
As an available printed circuit board surface protection material (hereinafter referred to as "surface protection material"), which is capable of protecting the copper or copper alloy circuit sections of printed circuit boards and holding the solderbility of the circuit sections, a compound having a benzimidazole derivative as an effective component is disclosed in Japanese Patent Application Kokai Publication No. Hei 5-186888. This surface protection material contains as the effective component 0.05 to 30% by weight of a compound represented by a formula: ##STR1## wherein X is a radical, with the number of carbon atoms from 1 to 7, of the group consisting of alkyl group, halogen atoms, amino group, lower diaminoalkyl group, hydroxyl group, lower alkoxyl group, cyano group, acetyl group, benzoyl group, carbamoyl group, formyl group, carboxyl groups, lower alkoxycarboxyl group and nitro group, n and p are integers of 0 to 4, and m is an integer of 1 to 10. Further, the composition contains 100 to 500 ppm of one or more heavy metal salts added as copper complex film coating formation promoter, e.g., copper formate, copper chloride, copper oxalate, copper acetate, copper phosphate, copper sulfate, manganese formate, manganese chloride, manganese oxalate, manganese sulfate, etc. The composition is provided as an organic or inorganic aqueous solution. A surface protection film having this surface protection material composition is formed in the manner similar to the usual coating film formation process. Specifically, the copper or copper alloy circuit surface of printed circuit board is finished by mechanical or chemical polishing, and then the printed circuit board is held dipped in an aqueous solution of surface protection material at 10.degree. to 50.degree. C. for 10 seconds to 10 minutes.
In the surface protection material of a compound with the above benzimidazole as the effective component, a metal salt of copper, manganese, zinc, etc. is added as complex film coating formation promoter for promoting cumulative formation reaction between the copper or copper alloy and the complex film. These metal salts act as catalyst with respect to gold plating layer for contact points and contact of the printed circuit board, and they form a complex film on the gold plating layer surface as well. (Gold is a metal having high oxidization and reduction potentials, and thus it is very stable and generally does not form a complex.) The complex film coating is an insulating film which is not uniform in its thickness. This gives rise to the problems of increasing the contact resistance and reducing the connection reliability of the contact points and the contact gold plating layer surface, and of damaging the appearance of the surface of the gold plating layer due to discoloring. Also, since there is a technical movement for avoiding flux washing of elements after mounting thereof (non-washing trend) because of a trend of abolishing the use of chlorine type organic solvents from the standpoint of environment safeguarding, it has become necessary to avoid the above problems.